Technical Field
Embodiments of the invention relate to ballast water treatment system and method onboard ships or sea-going vessels, which provide effective and continuous biofouling control by non-chemical processes.
Description of Related Art
Transportation of unwanted marine organisms including micro-organisms by ships' ballast water across the world has upset the ecological balance in many regions. As such, it has become a mandatory requirement by International Maritime Organization (IMO) to control the migration of marine organisms transported by ballast water worldwide especially the undesirable species.
Existing ballast water treatment systems may be broadly classified as treatment involving active substances and treatment without active substances. Treatment involving active substances requires the addition of chemicals to kill marine organisms in the ballast water, however the use of chemicals has negative impact on the environment. Treatment without active substances typically involves physical methods such as ultraviolet (UV) light disinfection and is more desirable due to its minimal impact on the environment. However, treatment without active substances has several major drawbacks including, but not limited to, the following:                1. Conventional UV systems without using active substance have a major drawback of high UV power consumption. Since existing filtration systems are unable to remove all the >50 μm organisms, the UV disinfection systems must use extremely high power to kill all the organisms from micro size bacteria to larger organisms including >50 μm organisms which have passed through the filters.        Furthermore, in high turbidity intake ballast water condition, the high turbidity will shield the effective UV irradiation from the UV light source and hence impede the water treatment efficacy.        As such, very high power consumption will be required by the conventional UV system for ballast water disinfection treatment. In most cases, to cater for the additional UV treatment power demand from the conventional UV ballast water treatment system, installation of additional generator is necessary, especially for ships with high ballasting capacities such as oil tankers, LNG carriers and bulk carriers. In most ships, installing additional generator onboard is not feasible due to space constraints and also economically non-viable. For this reason, many ship owners of large ballast capacity ships are unwilling to employ the more environment friendly UV ballast water treatment.        2. Most existing ballast water treatment systems employ a filtration system to remove larger particles and organisms before the filtered water undergoes UV disinfection. If the backwash water or the back flush water during ballasting process is discharged overboard at the same location where the ballast water was taken from, this will not violate IMO regulations. However, if the water was de-ballasted at another location different from the ballasting location, the back wash water cannot be discharged overboard, due to IMO regulations, as this water is considered to be of foreign origin. Some systems therefore bypass the filtration system but this inevitably compromises the de-ballasting performance. Some other systems increase UV capacity to compensate for the bypassing of filtration system but this further increases power demands. Yet other systems store the back wash water but due to the large volume of back wash water, the ship cargo carrying capacity is significantly reduced.        3. Biofouling is the undesirable accumulation or growth of microorganisms, plants, algae, or animals on wet or submerged artificial structures such as ship hulls and seawater intake pipes, and is a serious problem which can slow ships, block pipes, speed corrosion, and cause damage to the environment. Existing filtration and/or hydrocyclone systems are generally designed for removing particles or living organisms of size greater than 50 μm. However, IMO regulations also require control of living micro-organisms of sizes between 10 μm to 50 μm, which is critical and yet beyond the effective control range of the conventional hydro-cyclone or filter system for ballast water system. Since filtration systems are generally ineffective in removing the organism of sizes between 10 μm to 50 μm, existing ballast water physical treatment systems use UV light to kill living organisms in this size range. However, the 10 μm to 50 μm size organisms are much bigger than the bacteria and the use of UV to kill organisms in this size range will require extremely high power. As a result, the power consumption of UV system in existing ballast water treatment systems is typically very high.        UV light is able to kill or remove marine organisms and bacteria only at the point of contact. Beyond the point of contact, there is no control of the re-growth of the organisms and bacteria.        Living organisms and bacteria populating in submerged surfaces will colonize in biofilm which adhere to these surfaces. Existing UV light and conventional filtration system are unable to remove either the living organisms or the biofilm from these surfaces.        Many existing UV systems employ mercury vapour tubes which produce UV light when bombarded by electrons from the emitters in the tube or by the microwave generated externally which is sent through the magnetron into the tube. The mercury vapour tubes are prone to breakage due to their long-tube construction. In the event of breakage, the mercury vapour will leak into the ballast water and thereby polluting the environment.        Performance of UV light is adversely affected by water turbidity and is therefore much less effective on ships navigating in high turbidity waters.        4. During type approval test for any ballast water treatment system, the test site piping systems are disinfected and bio-fouling is cleansed before commencing the test. Many existing ballast water treatment systems are able to pass the type approval test simply due to the disinfected and cleaned bio-fouling system condition. Subsequently in actual shipboard practice, many of these typed-approved systems would fail the actual shipboard applications performance.        
Accordingly, a ballast water treatment system that eliminates the above and other problems of existing systems is highly desired.